The use of wind tunnels to test the aerodynamic characteristics of scale models, typically of aircraft, is well known. A model is mounted, via an internal balance or sting balance connected to an aft portion of the model, onto a fixed support. The performance of the aircraft under various simulated wind loading conditions is then examined by taking output readings from the internal balance.
In order to validate any results of wind tunnel testing, however, calibration of the balance prior to testing is required. Calibration of the balance involves mounting it onto a force adaptor, and applying, via the force adaptor, direct forces along and moments about, three predetermined mutually orthogonal axes.
Conventionally, the loading of the balance required for calibration is carried out by use of dead weights, mounted so as to apply forces of known magnitude in known directions. This method is traditionally a manual method and is thus very tedious, taking in the range 100-200 hours to perform. A further disadvantage of this method is that application of a number of different loading conditions simultaneously is limited.
Also known is the repositioning method of calibration, in which the directions of the applied force vectors are kept constant by a servo positioning system. This concept has been put into practice in a system located at the National Aeronautical Establishment Wind Tunnels Center in Canada. In this system, the load application point is levelled automatically in both pitch and roll.
Extension of this concept to a full six degrees of freedom system, as required in an internal balance calibration system, has been found to be very expensive and mechanically very complex. A complete discussion of this system is found in a report entitled `A Six Component Auto-Leveling Balance Calibration Frame`, by R. D. Galway, NAE Canada, Supersonic Tunnel Association, 58th meeting, 1982.
A further approach to the automation of internal balance calibration, known as the Master Balance Method, is by measuring calibration forces by use of another, more accurate balance. This method is free of the size limitations to which the internal balance is subject, and can be manufactured with accurate load cells in a stiff frame which minimizes interaction between forces acting along or about the three axes with respect to which the balance is calibrated. A disadvantage of this approach is the dependence on a further, external calibration system, resulting, inter alia, in a relatively expensive system.
Systems based on this approach have been developed in Germany and in Australia. This approach is discussed in a report entitled `A New Approach To Internal Strain Gage Balance Calibration`, by Bruce Fairlie of the Aeronautical Research Laboratory, Australia, and delivered at the Supersonic Tunnel Association, 69th meeting, 1988.
A non-repositioning approach to internal balance calibration is described in an article entitled `An Investigation Of Methods On Drain-Gage Wind Tunnel Balance Calibration For A Rig Without Repositioning After Loading`, by Han Buzhang, NAI China, Ingmar Johnson, FFA Sweden, and Zhao Lei, SARI China, and published in FFA Technical Notes, 1988-14. A built system is described in a report article entitled `A New Computer Controlled Rig For Calibration Of The T 1500 Wind Tunnel Balances`, by Gustav Ingmar Johnson of The Aeronautical Research Institute Of Sweden, presented at the 71st meeting of the Supersonic Tunnel Association, Apr. 3-4, 1989, Universal City, Calif., U.S.A.
In the non-repositioning approach, applied force vectors are allowed to move and their position relative to the balance axis is measured.
Among advantages of this system are mechanical simplicity, leading to low cost; and long term stability, due to calculation of resultant forces from stable and accurate parameters, facilitated by the use of stable and highly accurate load cells and position sensors.
Among disadvantages of the system built in Sweden, described in the article by Gustav Ingmar Johnson, are: (i) reliance on an assumption that no axial displacements occur in the system, (ii) friction at points where force actuators connect to the force adaptor, and (iii) a lengthy initial adjustment of the force actuators to the coordinate axes of the system.